The present invention relates to a magnetic resonance apparatus for acquiring information on the spins of a low sensitive nuclide of .sup.13 C, etc., for MRS (Magnetic Resonance Spectroscopy) after being enhanced through the utilization of a spin-spin coupling to .sup.1 H.
The main function of an MRI (Magnetic Research Imaging) is to noninvasively image the H.sub.2 O distribution in a living body of a human subject. This H.sub.2 O distribution provides the morphological information.
In contrast, the MRS is to detect .sup.1 H, .sup.13 C or .sup.31 C and investigate the metabolic function in a living body. Attention has now been paid to .sup.13 C -MRS. Since the .sup.13 C in nature reveals an abundance ratio of as low as 1.1% it can be used as a tracer.
For the conventional .sup.13 C -MRS, however, a problem arises from the low S/N. In order to overcome this problem, a method has been proposed which utilizes the polarization of .sup.1 H. The method is classified into a .sup.13 C observation method for transferring the polarization of .sup.1 H to .sup.13 C and observing a signal from the .sup.13 C and an .sup.1 H observation method for, after transferring the polarization of .sup.1 H to .sup.13 C, returning the polarization back to the .sup.1 H and observing a signal from the .sup.1 H.
As the .sup.13 C observation method, an INEPT (Insensitive Neclei Enhanced by Polarization Transfer) has been announced in G. A. Morris and et al "J. Am. Chem. Soc., Vol. 101, p.760, 1979. FIG. 1 shows an RF pulse sequence of the INEPT.
As the .sup.1 H observation method, an HSQC (Heteronuclear Single Quantum Coherence) has been announced in G. Bodenhausen "Chem. Phys. Letters, Vol. 69, p.189, 1980". FIG. 2 shows an RF pulse sequence of the HSQC. The RF pulse of the HSQC is so designed as to return, back to .sup.1 H side, the polarization which has been transferred from .sup.1 H to .sup.13 C by INEPT.
A RF pulse for bringing to a transverse magnetization, the spins of a nuclide of interest is referred to as an excitation pulse, an RF pulse for refocusing the spins of a nuclide of interest is referred to as a refocusing pulse, and an RF pulse for inverting the spins of a nuclide of interest is referred to as an inversion pulse and an RF pulse for returning a transverse magnetization to a longitudinal magnetization is referred to as a return pulse. The excitation pulse is constituted by an RF pulse having such a function as to allow a transverse magnetic component to be generated at the spins of the nuclide of interest. The spins to which the excitation pulse is applied flip, for example, at 90.degree. about an x- or y-axis. The spins to which the refocusing pulse is applied are rotated, for example, 180.degree. about an x- or y-axis. The inversion pulse is constituted by an RF pulse having a function to invert the polarities of the spins in a longitudinally magnetized position. The spins to which the inversion pulse is applied are rotated, for example, about the x- or y-axis.
In the INEPT, the excitation pulse is applied to .sup.1 H. After 1/(4.multidot.J) from the application of the excitation pulse a refocusing pulse is applied to the .sup.1 H and, simultaneously with the refocusing pulse, an inversion pulse is applied to .sup.13 C and, by doing so, a coherence state "2IxSz" is generated at an echo time, where I corresponds to the spins of .sup.1 H and S corresponds to the spins of .sup.13 C.
Further, at the echo time, the return pulse is applied to the .sup.1 H. In this way, a state "2IzSz" is generated. Simultaneously with this excitation pulse, the excitation pulse is applied to .sup.13 C and a coherence state "2IzSx" is set. The coherence "2IzSx" represents the spins of .sup.1 H in the longitudinal magnetization position not producing any signal and the spins of .sup.13 C in the transverse magnetization producing a signal. By doing so, a signal enhanced by the polarization transfer can be detected from the .sup.13 C.
The magnitude of the signal can be given by 2IxSz.multidot.cos.theta., provided that the actual flip angle of .sup.13 C produced by the inversion pulse is given as .theta.. That is, achieving the spins of .sup.13 C accurately at 180.degree. through the application of the inversion pulse to .sup.13 C is important to obtain a high S/N ratio.
For the NMR spectrometer, because of the sample size being small it is possible to reduce the radius of a probe (RF coil) and, without the need to apply too much power to the probe, the spins of .sup.13 C are effectively inverted through the application of the inversion pulse. Even if, therefore, the width of the inversion pulse is of the order of a few tens of microseconds, the spins of .sup.13 C are inverted with neither an excess nor a shortage.
For the in-vivo case of a human being in particular, the probe size becomes greater and, in order to invert the spins of the .sup.13 C, a larger power is necessary. Since, however, a restriction is placed on the withstand voltage of a capacitor of the probe, it is not possible to apply too much power to the probe for a shorter period of time. In order to accurately invert the spins of .sup.13 C, it is necessary to lengthen the pulse width of the inversion pulse.
Since the longer the pulse width, the narrower the frequency band, the spins of the .sup.13 C of these .sup.13 C compounds having a chemical shift of a few KHz cannot all adequately be inverted.
Here it is supposed that an observation is made on the action of the .sup.13 C by administering, to a living body, the glucose C-1 labeled with the .sup.13 C. The difference between the resonance frequency of the .sup.13 C in the 1-position of the administered glucose C-1 and that of the .sup.13 C in an amino acid produced through the metabolic process in the living body is about 70 ppm. The 70 ppm corresponds to 1.5 kHz at 2 teslas. After the width of the inversion pulse for the .sup.13 C is set, if the center frequency of the inversion pulse is matched to the resonance frequency of the .sup.13 C in a amino acid C-4, the spins of .sup.13 C in the glucose .alpha. C-1, glucose .beta. C-1 flip only at about 60.degree., that is, there is hardly no polarization transfer between the spins of .sup.1 H and those of .sup.13 C in the glucose .alpha. C-1, glucose .beta. C-1.